The present invention relates to a means for preventing undesirable transfer caused by offsetting phenomenon which occurs in a fixing device of an electrophotographic apparatus, and relates to a means for giving gloss to a color image provided by electrophotography.
In a fixing process for a toner image formed by means of electrophotography, a heating roller fixing method is most commonly adopted. In recent electrophotographic apparatus by which color images are copied, it is required to fuse toner completely in order to ensure color reproducibility and give a sufficient gloss to an image, or in order to ensure color transmission when the transferred sheet is to be used in an OHP. When fixing is performed, using a widely known toner made of linear polyester under the condition satisfying the aforementioned matters, it is essential to apply a large amount of silicone oil of low viscosity as a releasing agent in order to prevent offsetting phenomenon from occurring at high temperature.
When a large amount of silicone oil is used in the manner mentioned above, various problems are caused such as spillage of silicone oil, adhesion of silicone oil to transfer sheets, and the like. Especially when an obtained image is used in an OHP, the applied silicone oil is not sufficiently impregnated into a transfer sheet, so that the operator's hands become dirty easily, and the transmission property of OHP sheets is deteriorated remarkably. Therefore, a toner in which common polyester is used, in other words, a toner composed of wax, has been adopted. However, a sufficient toner fusing condition can not be obtained at present. As described above, in the case of a color toner image, a glossy surface is often required. According to the prior art, in order to obtain a glossy surface, a large amount of releasing agent is coated as described above, or an image once fixed is fixed again by an optional unit in the case of a common polyester toner, that is, a toner composed of wax.
In order to avoid this inconvenience, toner in fused condition is cooled in such a manner that: an endless belt is provided between a roller, which is one of a pair of fixing rollers, and a conveyance roller; a developed transfer sheet passes around the fixing roller and the conveyance roller; and the fused toner is cooled and solidified onto the surface of the transfer sheet while it passed between the rollers. In this case, it is necessary to appropriately position the pair of rollers and adjust the temperatures of the belt and conveyance roller. In order to realize this, it is necessary to provide a precise sensor to detect and control the temperature. However, when the temperature sensor is directly contacted with the belt, the formed toner image is damaged, and dust or toner is adhered onto the sensor, so that a faulty operation of the sensor is caused and control can not be performed sufficiently.
It is the first object of the present invention to solve these problems and provide a fixing device in which a releasing agent such as silicone oil is not used at all, or an extremely small amount of releasing agent is used, and in which toner fusion is sufficiently ensured after the fixing operation, so that color reproducibility is excellent. Further, the first object of the present invention is to accomplish a fixing operation in which a glossy color image can be obtained and transmission by an OHP is excellent, and further, color reproducibility is improved.
A thermal fixing device for a toner image, especially a fixing device in which fixing is conducted through a belt, is disclosed in the official gazette of the Japanese Patent Application Open to Public Inspection No. 36342/1974. This device is structured in such a manner that: a heating roller is encircled by a belt; a pressure roller is disposed in opposition to the heating roller; and a transfer sheet on which a toner image is formed, is pinched and conveyed between the pressure roller and the belt so that fixing can be conducted. In this case, however, sufficient nip region N.sub.1 can not be obtained. Further, a sufficient amount of heat can not be supplied from the heating roller to the toner through the belt. Therefore, toner fusion is not sufficient to obtain a glossy surface of the image.
A fixing device by which fixing is conducted through a belt, is also disclosed in the official gazette of the Japanese Patent Application Open to Public Inspection No. 122665/1986. In this case, also, the length of nip region N.sub.2 formed by the belt with regard to the pressure roller, is not sufficient. Consequently, a sufficient period of time can not be obtained for fusing, so that heating of the transfer sheet is not sufficient. Accordingly, the fixing property depends on the material and thickness of the transfer sheet, and the thermal fusing operation is not stable.
The second object of the present is to provide a fixing device by which glossy fixing can be stably performed, wherein the thermal fixing property of a toner image does not depend on the material and thickness of a transfer sheet, and by which a color toner image of high quality can be obtained without causing offsetting phenomenon in the transfer operation.